Piezoelectric device



June 1o, 1941.

C. K. GRAVLEY ETAL PIEZOELECTRIC DEVICE Filed Sept. i/'1939 2 Sheets-Sheatl wlw.

June 10, 1941.

c. K.-GRAVLEY ErAL 2,244,690

PIEZOELECTRIC DEVICE Filed Sept. 1l, 1939 2 Sheets-Sheet 2 8 5o vozr Q 6 APPL/0 G l Y H65. f 4

o /5 zo es so I a5 wPf/PAn/kfawr/65H05 I9 "C 7/ 2J 1 26,0 /5 Q ,4 I4 el ,o N l 0 /cvo 20o Sco 400 500 ,4P/2050 Vanaf l U V Patented `'une 10, 1941 PIEZOELECTBIC DEVI CharlesLGravleyandJoseph J.Nell,Cieveland, Ohio, assignors to The Brush Development Company, Cleveland, Ohio, a corporation of OhioA Application September 11, 1939, Serial )10.294.237 1s claim. (CL 1v1-sav) This invention relatesto piezoelectric appara-l tus utilizing piezoelectric units for interconverting electrical and mechanical vibratory energy, and relates particularly to heated piezoelectric units used for the purpose of improving the operating characteristics of such apparatus.

It is known that piezoelectric units oi' the Rochelle salt type exhibit certain technical characteristics (described below in detail) which become especially objectionable under some conditions of usekand which generally impair the use of such units in piezoelectric apparatus.

It is the primary object of this invention to provide a piezoelectric transducer assembly in which these objectionable features are substantially avoided. y

It is another object of the invention to pro- A vide a transducer in which objectionable operating characteristics resulting from temperature variations are substantially avoided.

It is another object to provide a transducer assembly utilizing a heated piezoelectric unit.

It is another object to provide a transducer assembly having a heated piezoelectric unit combined with heated damping means.

It is another object to provide a piezoelectric motor'device utilizing a heated piezoelectric unit in which overloadingis prevented while the unit is being heated to its operating temperature; Other objects will in part be obvious land will in part appear hereafter.

lIt has been found that piezoelectric materials of the Rochelle salt type exhibit a number of technical characteristics. which vary with or are affected by temperature changes. Some of these characteristics change only to a small expoint lies within the temperaturefrange in which most devices are operated., Consequently, the abrupt changes in characteristics which occur in this upper critical temperature range may,

and often do, occur while the device is in operation, causing unexpected and objectionable alterations in its performance. This is especially true of devices using unrestrained piezoelectric' members. The bimorph type of piezoelectric unit wherein the piezoelectric material is restrained by other piezoelectric material or by non-piezol morph or over the large effects observed in untent with changes in temperature while others change greatly. but it has been found that all oi' them are subjectto rather abrupt and sharp changes when the temperature passes through either one of two critical ranges centered around temperatures known as the Curie'points of the material. Ihe Curie points arezsenerally spaced apart somewhat in the temperature scale and for that reason are commonly referred to as the "upper and vlower'f Curie points. For example, the upper Curie point of Rochelle salt is commonly considered to be around 23.5 C., while the lower Curie point occurs at about 20 C. It will be apparent that in general, the lower Curie point ot Rochelle salt is so far below the temperaturesv at which piezoelectric devices are ordinarily operated that it seldom needs to be given any consideration. p

On the contrary, however, the upper Curie electric material greatly reduces these undesirable characteristics, so much in fact, that they may be considered negligible for many applications. However, in other applications the remaining effects are of importance. Heretofore. little. if any. control could be exercised over these remaining objectionable eilects in the birestrained plates (except by substituting bimorphs when possible) with the result that they 4have come to be regarded practically as inherent defects of piezoelectric materials which could not be avoided commercially by any practical means.'

As intimated, however, this invention provides such commerciallypractical means and permits a high degree oi control for both bimorph and unrestrained piezoelectric transducers.

The essence of this invention liesin the discovery'that highly satisfactory perfomance may l be obtained from a piezoelectric unit when it is heated to temperatures somewhat above the upper Curie point, and particularly when its temperature is held within a moderately restricted range lying somewhat above the Curie point. It has been found particularly that heating the piezoelectric unit to these temperatures aifects each of the technical characteristics known as sensitivity," linearity," hysteresis," and impedance; stabilizing sensitivity and impedance, improving linearity, and materially reducing creep and hysteresis.

By the term sensitivity" is meant the deilection, per unit of applied voltage, of a chosen point on a pierpelectric'unit. Speaking genertemperature rises above it, the sensitivity gradually becomes stabilized at a low level,

changing but slightly as the temperature increases further.

The term linearity expresses qualitatively a relation of direct proportionality between the deflections of a piezoelectric unit, and the corresponding applied voltages which produce those deections. It will be understood that if there is direct proportionality between these values, then the sensitivity curve will be a straight line,

and will be said to be linear. Perfect linearity in a piezoelectric unit is the ultimate desire since in terms of operation, it means that if the applied voltage is doubled, for example, the deiiection of the unit will also be doubled. Furthermore, it is especially desirable that perfect linearity be exhibited over a wide range of applied voltages since then the same relative accuracy may be obtained from a device at high values of applied voltage as at low values, and the field of utility of the device will be correspondingly widened. It has been found that at temperatures below the Curie point, the relationship between deection and voltage is non-linear, while at temperatures above the Curie point, the relationship is substantially linear.

Creep is a term Aused to describe an especially objectionable characteristic of piezoelectric materials. A piezoelectric unit exhibiting this phenomenon rapidly assumes an initial deiiection when a voltage is applied to its terminals, but then it gradually creeps to greater deflections until it ultimately comes to rest at a maximum value. When the charge is removed from the unit, the same type of action reoccurs in the reverse direction. It has been ,found that this creep action attains greatest magnitude, and is therefore most serious, at temperatures below the upper critical temperature range, 'but is negligible at temperatures above the critical range.

Hysteresis in piezoelectric material is fully the term being used' to signify that the ultimate deflection of the unit lags behind the applied voltage when the voltages are applied in a reversed cycle. In terms of actual operation, if a unit is permitted to assume its ultimate deilection when a constant positive diierence of potential is applied to its terminals, a greater negative diierence of potential must be applied in order to restore the unit to zero deflection. Hysteresis has been found to occur to an objectionable extent in unrestrained piezoelectric material at temperatures below the upper Curie point, but to become negligible at temperatures above the Curie point.

The electrical impedance of a piezoelectric devicek varies markedly with temperature. In general, the impedance at any particular frequency first decreases as the temperature increases, reaching a minimum in the neighborhood of the uppenCurie point; and then increases as the temperature is raised above the Curie point, increasing rapidly at rst in the critical range, and then more slowly as the temperature is analogous to hysteresis in magnetic materials,

further increased. The advantage oi operating' thepiezoelectric unit above the Curie point, in so far as impedance is concerned, lies in the fact that variations in impedance for any specified frequency are greatly minimized, especially if the temperature is maintained well above the.v

the circuit will be essentially constant and therefore may be predicted.

From what has been said above.it will be understood that a piezoelectric unit heated to above the upper Curie point will exhibit negligible creep and hysteresis, especially good linearity, and stabilized -sensitivity and impedance. However, a more comprehensive understanding of the improvements resulting from the use of a heated piezoelectric unit may be had through reference to a particular device embodying the invention, a pen recording device having been chosen for purposes of illustration.

Referring now to the drawings,

Fig. 1 is a plan view of a piezoelectricallyactuated pen recording device.

Fig. 2 is a front elevation of the device having portions thereof broken away to show interior features.

Fig. 3 is a sectional view taken onthe lines 3, 3 of Figs. 1 and 2.

Fig. 4 is an exp1oded view of the heating unit employed in the device of Figs. 1 through 3.

Fig. 5 is a chart showing the eiect of temperature on the sensitivity of a piezoelectric unit of the type used in the device of Figs. l through 3.

Fig. 6 is a chart showing the effect of temperature on thelinearity of the response of a piezoelectric unit.

Fig.. 7 is a diagrammatic representation of a test voltage applied to the device of Figs. 1 through 3.

Fig. 8 is a typical curve illustrating the manner in which the device of Figs; 1 through 3 responds to the test voltage of Fig. 7 when the temperature of the device is below the upper Curie point.

Fig. 9 is a response curve similar to that of Fig, 7, the temperature of the device being above the upper Curie point, but the adjustment of the damping unit of the device remaining unchanged.

Fig. 10 is a response curve similar-to that of Fig. 8, the damping unit having been readjusted to produce optimum damping at the higher temperature.

Fig. 11 is a schematic diagram of a suitable circuit for the device of Figs. 1 through 3. Referring now to Figs. 1, 2 and 3, the pen recording device comprises a piezoelectric motor assembly A actuating a motion-amplifying assembly B which carries a pen adapted to oscillate transversely back and forth across a paper strip C travelling longitudinally under the pen. A damping unit D is coupled to the motion-amplifying assembly B. Electric heating units E, E supply heat to the piezoelectric motor and to the damping unit, and a thermostat F connected in circuit with the heating units E, E controls the heating units to maintain the temperature of the device at a desired level.

The piezoelectric motor assembly A comprises a cup-shaped case I composed preferably of cast.

metal, the cavity in the case being closed by a metal cover plate 2 secured tightly to the case in a suitable manner as by means of screws 3, 3 and clamping a pair of gaskets I, 4 between the abutting surfaces of the plate and case. A square multiple-plate piezoelectric unit 5 of the bimorph type (such as described in U. S. Patent No. 2,105,011 granted to A. L. W. Williams, or U. S. Reissue Patents No. 20,213 and 20,680 granted to C. B. Sawyer) is clamped at each of three corners between the'cover plate and the base wall of the case, the piezoelectric unit being spaced from the case by rubber pads 6. 6 and from the cover plate by ball bearing supports 1, 1, of the type described and claimed in the copending appll' cation of Alfred L. W. Williams, Serial No. 240,261- filed Nov. 18, 1938. 'Ihe fourth and unclamped `corner of the piezoelectric unit has a driving clamp l fitted therearound: and secured thereto, a drive pin 9 being secured to the clamp and extending outwardly therefrom through an opening in the cover plate 2. -The case I is provided with a contact strip I recessed in a side wall thereof and carrying four contact terminals designated II, IIa, I 2, and I2a. 'Ihe electric leads of piezoelectric unit pass between gaskets 4, 4, and are connected to terminals II and IIa. The heating circuit (described below, and comprising the heating units E, E' and thermostat F) is connected to the other terminals I2 and I2a. It will be understood that an alternating potential applied between terminals Il and IIa causes piezoelectric unit 5 to vibrate in such manner that drive pin 9 is vibrated in an endwise direction.

The motion-amplifying assembly B is secured to and supported by a bracket I3 and comprises a spindle I4 which has two curved bearing surfaces of the same radius and another curved surface of slightly larger radius disposed between the two first-mentioned surfaces. The spindle is operatively supported by flexible tension members I5, I5 so that it may roll back and forth a small distance along the plane defined by those members. is frictionally mounted on the middle curved surface and connected to drive pin 3. A long hollow tube I1 terminating in a glass pen point I8 is secured to thel top of the spindle, and is supplied with ink through tube I3 from inkwell 20. Due to the difference in length of the radii of the curved bearing surfaces and the middle curved surface, slight endwise motion of drive pin 9 in response to an applied electromotive force between terminals II and I la results in a relatively large angular motion of the spindle I4 and of the pen point I8.

In operation, a strip of paper C moves over roller 2l and table 22 under pen point I8. The pen point traces a graph on the moving paper of the electrical waves applied between the terminals. Fora detailed description of the operation of this piezoelectrically actuated pen recording device, reference may be made to U. S. Patent No. 2,149,216 granted to Charles K. Gravley.`

The damping unit D is of the tuned or frequency-selective type and comprises an elongated =bar 23 of compliant and exible damping material such as Viscoloid or otherv similar rubber-like material. The upper end of the bar 23 is secured to the lower end of the spindle I4, while the lower portion of the bar 23 is held in a clamp 24 secured to cover plate 2. Two metal discs 25 are frictionally retained at intermediate points along the :bar 23, and may be adiustably positioned therealong so that their spacing from each other and from the upper end of the bar is eifective in producing optimum damping at a desired frequency. The operation of this type of damping unit is described in detail and claimed in the copending application of A. L. W. Williams and Joseph J. Neil, Serial No. 240,260, filed Nov. 14, 1938, and reference is made to that application for a full disclosure of the damping unit shown here and of various modifications which may also be us'ed.

The heating units E, E' are disposed adjacent the remote faces of the piezoelectric motor assembly A. Each unit consists of a at helix of Aresistance wire or ribbon 28 wound around a A similar flexible'tension member I5 f between the ends of adjacent coils.

`of the helix are connected to external lead wires 23, 29' by suitable means such as rivets 3l, 35. Insulating sheets 3|, 3| are secured to sheet 21 on opposite sides of the helix, the assembly being secured together at the top and bottom by suitable means such as rivets inserted through the holes 32, 32.

Heating unit E is disposed adjacent the outer face of the base wall of case I being surrounded -by a gasket 33 having slightly greater thickness. An outer cover plate 34 protects the heating unit, the plate being secured to case I by means of screws. In order to hold the heating unit in/ spaced relation to gasket 33, and also to insulate -the heating unit thermally from the outer cover plate 34, a compressible layer of insulating material 35 such as woven glass or asbestos cloth is placed between the outer surface of the heating unit and the inner surface of cover plate 34, its thickness being such that it provides the desired insulating effect and also presses firmly against the heating unit when the cover plate 34 is tightened against gaskets 33.

Heating unit E is disposed adjacent the outer surface of cover plate 2, being surrounded on three sides by a U-shaped gasket 35 and being partially covered with thermally-insulating cloth 31. cover plate 38 which is secured by screws 33, 33 to case I. This latter cover plate does not extend over the whole exposed surface of cover plate 2, the right hand portion, except the upper part forming bracket I3, being bent outwardly to form two walls 40 and 4I of a housing for the ymotion-converting lassembly B and its attached damping unit D. A second -bent plate 42 having a. U-shaped section forms the top wall 42a, side wall 43, and bottom wall 44, the plate 42 being secured to wall 4I by means of screws 45, 45. A rectangular plate 45 closes the upper portion of the housing which lies directly below bracket I3. It will be seen that this assembly of plates forms a housing which substantially encloses the assembly B and damping unit D.

The thermostat F is of conventional construction, and since its structure forms no part of this invention, description of its internal features is omitted. Its action will be fully understood by those skilled in the art to which this invention relates. 'I'he thermostat is disposed within the housing around the damping unit, being clamped into the corner formed by walls 4l and 4I by means of la clarmp 41, which is secured to Wall 45 by means of bolt 43. The thermostat is connected in series circuit with heating units E and E', lead wire 23 of heating unit E being connected to terminal 4I of the thermostat, lead wire 23' of heating unit E being connected to its other terminal 5l. Iead wire 23' of heating unit E' is connected to terminal I2, and lead wire 2l of'heating unit E is connected to terminal Iza. 'Ihe thermostat is preferably of the adjustable type so that it may be set to open or close the heating circuit when the temperature within the housing rises above or falls below a desired value or restricted range `of values. It will be recognized that the thermostat does not respond directly to the temperature of the piezoelectric unit 5 since it is spaced some distance away. It will also be recognized, however, that It is clamped in place and protected by av the temperature gradient between the piezoelectric unit and the thermostat remains practically uniform over the range of temperatures which are ordinarily employed in the device. For that reason it may be necessary to adjust the thermostat to respond to a temperature somewhat different from the temperature which it is desired the piezoelectric unit should attain during operation, but once the adjustment has been made, the thermostat will thereafter maintain the piezoelectric unit at the desired temperature.

'I'he structure of the device having now been described, it will be understood that when it is `desired to operate the device, the electromotive force which it is desired to record is connected to terminals H and Ila, while a suitable source of heating current is connected to terminals l2 and I2a. After the piezoelectric unit 5 and t damping unit D have attained their respective thermos-tatically-controlled temperatures. .the recording device will trace an accurate record of the electromotive force connected to terminals Il and I'Ia.

Some of the advantages which result in the pen-recording device from the use of a heated piezoelectric unit either by itself or in combination with a heated damping unit will be more apparent from a study of the curves shown in Figs. 5 through 10.

Fig. 5 is a graph showing the variation with temperature of the sensitivity of a piezoelectric unit of the type shown in Fig. 3 and designated by the numeral 5. The curve shows the deflections produced at various temperatures by a constant applied electromotive force of 50 volts. It

will be noted that when the temperature of the slightly with further increases in temperature.

It will be noted that at approximately 30 C., volts produces a deflection of about 2 units, from which fact it will be apparent that operation of the piezoelectric unit at temperatures above the Curie point results in marked reductions in sensitivity. This apparent disadvantage, however, is more than offset by the attendant improvements in operation with respect to linearity, creep, etc., as will be shown below.

The improvement in linearity obtainable by operation of a piezoelectric unit of -the type shown in Figs. 1 to 3 at temperatures above the upper Curie point is clearly shown in Fig. 6, in which deflections are plotted against applied voltage, each curve in the group representing the relationship existing between deflection and voltage at' the indicated temperature. At temperatures of '14 C. and 19 C. it will be noted that there is a relationship of approximate direct proportionality for voltages above about 25, but that the relationship is non-linear for lower voltages. At 23 C. the relationship between deflection and applied voltage is extremely non-linear, while at 28 C., there is a linear relationship up to voltages of about 250, the relationship then becoming very slightly non-linear for higher voltages. At 32 C.. it will be noted that there is -75 practically perfect linearity between deilection and voltage up to voltages as great as 500. The merit of a linear relationship will perhaps be understood better by realizing that in terms of operation, it means that if a voltage of volts produces 3 units deflection, then a voltage of 500 volts will produce a deflection ve times as great, or 15 units of deflection. Conversely, if the recording pen is deflected 3 units by an applied voltage of 100 volts, and is deflected 15 units by a second applied voltage of unknown value, the operator knows that the second applied voltage is five times as great, or is 500 volts. On the other hand, if the device is being operated at a temperature at which the relationship between deflection and voltage is non-linear, the operator must refer all deflections to a calibration curve in order to determine what voltage corresponds to a given deflection. In short, with a linear relationship, the record lproduced on the moving paper may be read and understood directly; whereas with a non-linear relationship, the record must, in effect, be translated.

The effect on creep of operation at temperat tures above the upper Curie point may be understood by comparing Figs. 9 and 10 with Fig. 8. As indicated above, the phenomenon of creep occurs predominantly at temperatures below the upper Curie point and may be substantially avoided by operating the piezoelectric unit at temperatures above the Curie point. This state of facts is shown by Figs. 8, 9, andI 10 which are the graphical records produced by the pen recording device at different temperatures when the voltage which is applied to the device has the characteristics illustrated by Fig. '7. In this latter figure, the horizontal axis corresponds to zero voltage, projections above the axis representing voltages of one sign, for example, positive, While projections below the axis represent voltages of the opposite sign, or negative. Time is measured horizontally along the axis. Fig. 7 shows that, starting from the left end, no voltage was applied to thedevice for a period of time extending to point K. At that moment the voltage Was increased valmost instantaneously to a positive value represented by the length of vertical line K-L. lI'his value of voltage Was then maintained for the time interval corresponding ,to the.length of the horizontal line L M, when it was reduced almost instantaneously to zero. No'voltage was applied to the device for the time interval represented by the horizontal distance N-P, but at the moment designated by letter P, the voltage was decreased almost instantaneously to the negative value represented by vertical line P-Q. This negative value of voltage was maintained constant for the time interval Q-R and then increased to zero, no voltage being applied for the time interval S-T. A similar series of voltage changes was then applied, the time intervals U-V, W--X, and Y-Z being somewhat shorter in the latter series than their corresponding intervals L M, N-P, and Q-R in the first series.

Fig. 8 represents the type of record obtained from the pen recording device when the temperature of the device as a. Whole lies below the upper Curie point and remains constant throughout the duration of the test, the damping unitV D having been adjusted for optimum damping at the particular temperature of the test. The curve shown in the figure was obtained at a measured temperature of about 15 C. It will be seen that the record obtained under these conditions is'far from'being a duplicate of the curveshownln Fig. '1. various portions which should contain sharp corners being rounded ofi'. This rounding-oi! represents the effect of creep. Again following the curve from the left end, it will be seen that when the voltage was suddenly increased from Kto L, the pen point did not move a corresponding amount all at once. Instead it moved rapidly from K' to L' under the influence of the sudden change of applied voltage,

' but then crept upward to larger deections during the time interval L.M. this creeping action appearing in Fig. 8 as the asymptotically curved portion L'M. At the moment M, when the voltage was suddenly reduced from Mto N, y

the pen point again executed the same type of motion but in the reverse direction. moving quickly under the influence of the sudden voltage reduction from M' to N' and then gradually creeping toward the horizontal axis. 'I'he negative voltage cycle P-Q'-R-S produces a similar type of response in the pen, the curve PQ' R'S'T' being an inverted reflection of the Under the influence of the series of voltage changes designated T-U-V-W-X-Y-Z, pen again executed similar motions wherein .creep caused rounding of the corners, the dif` ference being that in this 'series of changes. the time intervals between succeeding changes were short enough to prevent the pen from creeping to the full value of deflection which it would otherwise reach. This is shown in the record at the point X', where it will be seen that the chart is displaced slightly from the axis instead of lying on the axis as it did in the first series at the point P'.

The curve of Fig. 9 was obtained from the same device used in the test of Fig. 8, after it had been warmed from 15 C. to above the Curie point, its actual temperature being about 32 C. In this test the damping unit was left in its previous adjustment which, as indicated above, produced optimum damping at about l C. It will be understood .that since the damping unit contains material whose properties vary with temperature. the adjustment for optimum damping at the temperature used in the test of Fig. 8 will no longer be the adjustment which produces optimum damping at the higher temperature used in this test. This fact is illustrated partly in Fig. 9. where the improper adjustment of the damping unit is reilected by the overshoot" and train of vibrations occurring at each of the points LH Nn Qn s". UH. WH and Y". be understood that the damping unit, being out of adjustment for the higher temperature, no longer absorbs enough energy from the vibrating pen system to prevent it from travelling beyonddeflection represented by the horizontal portieril following each of the areas of overshoot."

Disregarding the overshot areas of the curve in Flg..9 it will be observed that there are no asymptotically curved portions characteristic of -creep such as appear in Fig. 8.. The higher operating temperature has-been effective in eliminating these effects of the creep phenomenon. This will be more apparent in Fig. 10, where the overshot portions have been eliminated by readjusting the dampingunit to produce optimum damping at the higher temperature, the test being otherwise conducted under the same conditions which prevailed in the test of Fig. 9. It will be seen from the curve of Fig. that the pen can be caused faithfully to follow the voltage changes of Fig. 7, the rounding-oil due to creep a having been eliminated by operation at the higher temperature, so that the record obtained from the device is an accuratechart of the voltages applied to the device.

As indicated above, the results shown in Fig. l0 were obtained by operating the device of Figs. i through 3 at a temperature of about 32 C. The same kindof results may be obtained over quite a range of temperatures, however. Temperatures between about 30 C. and 45 C. are preferred, particularly since the sensitivity curve within this rangeis linear, and'since the sensitivity undergoes only slight variation with temperature. It should be understood, however, from what has been said above, that temperatures below 30 C., but above the Curie point, may also produce the same kind, but a leser degree. of the improvements in operation which form the basis of this invention, since by heating the piezoelectric material to only slightly above the Curie point, creep and hysteresis may be markedly reduced, linearity may be materially improved, and impedance variations may be minimized. It will be understood that the piezoelectric unit should not be heated to a temperaturevhigh enough to destroy its piezoelectric properties. For Rochelle salt the upper limit is about 55 C.

A further advantage of the heated piezoelectric unit which is of importance mainly in connection with piezoelectric motor devices is the overload protection that is possible with suitably designed circuits. For an uncontrolled piezoelectric device the sensitivity and electrical impedance of the piezoelectric unit will vary with temperature to a marked extent, as pointed out above. In general, an amplifier or .other source of signal for an uncontrolled piezoelectric device must have an available output which is several times the output necessary for operation at room temperatures in order that there will be enough output for operation at high temperatures. At room temperature, the impedance of the piezoelectric device is relatively low, and consequently the impedance of the amplier or other source must also be relatively low so that satisfactory high frequency response may be obtained. Normally the output of the amplifier is adjusted to a satis; factory valu'e for the temperature at the time of operation. However. if the output control of the low impedance source is accidentally turned on full when the piezoelectric unit is at room temperature, the excessive output may very likely damage the piezoelectricunit. All of this high output is necessary, however, when the pieroelectric unit has a higher temperature.

On the other hand, a piezoelectric unit heated to about 30 C. or higher when in operation has a much higher impedance than at room temperature, and the sensitivity'is greatly reduced. 'I'he amplifier or other source for the heated piezoelectric unit may be designed to supply enough output to operate the unit satisfactorily at its heated temperature, but not much more. so that there is no danger of damaging the unit during normal operation. Due to the relatively high impedance oi theunit at its operating temperature, the amplifier may also have a relatively highlmpedance..Nowiftheoutputcontrolof the ampliiler is accidentally turned on full before the unit has been heated to its operating temperature, the amplifier will tend to supply a damaging output, but since the crystal impedance at such temperature is relatively low, and the amplifier impedance is relatively high, only a fraction of the normally-available output will be delivered to the piezoelectric unit, the remainder appearing across the internal impedance of the amplifier, and the piezoelectric unit is thus protected from damage due to excessive voltage. I'he frequency versus amplitude characteristic will not be correct at low temperatures, but since the device is intended for use only when heated, this is of no consequence.

By way of example, reference is made to Fig. 11, in which ll is a vacuum tube, 52 is a plate supply battery, B3 is a plate coupling resistor, 54 is the piezoelectric unit, and I5 is a blocking condenser to prevent application of a D. C. bias 4 to the piezoelectric unit. The impedance of the piezoelectric unit at one frequency may be 160,000 ohms at 30 C., and 80,000 ohms at 23 C. Two hundred volts may be required for satisfactory operation of the piezoelectric device at 30 C., and only 50 volts at 23 C. At 30 C. 800 volts may produce the maximum safe displacement of the unit, and at 23 C. 200 volts may produce the maximum safe displacement. If the plate resistance of the vacuum tube is 200,000 ohms, and the coupling resistance 53 is 200,000 ohms, the effective source resistance R' will be 100,000 ohms. 'I'he vacuum tube Il and plate supply battery l2 may be so chosen that the maximum effective generated voltage in series with R will be about 300 volts. The maximum voltage available across the piezoelectric unit, assuming its impedance :c to be that of a pure capacity, will be :v Swami-100,000 Thus at 30 C. the maximum available voltage at the piezoelectric terminals will be v or a little more than necessary for satisfactory operation, but not enough to damage the unit; and at 23 C., the maximum available voltage at the piezoelectricunit terminals is which is more than necessary for satisfactory operation, but not enough to damage the unit.

0n the otherr hand, if the amplifier had an effective impedance of, for instance,10,000 ohms instead of 100,000 ohms, nearly' the full 300 volts could be applied to the piezoelectric unit. and this would be sufficient to damage the unit at room temperature.

As indicated in the preceding paragraphs, changes in temperature not only affect the properties of the piezoelectric unit, but also anect the properties oi' damping materials sumciently to produce undesirable alterations in their performance. It consequently is advantageous to stabilize the damping characteristics of a unit by heating it to temperatures in excess of any to which it might be subjected while in operation. F

This may be accomplished in the manner shown in the device of Figs. 1 to 3 where a common source of heat is provided for the piezoelectric unit A and the tuned damping unit D, or it may be accomplished by providing a separate thermostatically controlled source of heat for the damping unit. It will be apparent, of course, that thermostatic control is generally desirable as a means for holding the temperature of the damping unit within relatively narrow limits at the desired temperature level, and in view of what has been said above, it will be recognized that such control is especially desirable in connection with tuned or frequency selective devices which, by their nature, are easily thrown out of adjustment by temperature variations. It will also be understood, however, that thermostatic control over the temperature of the damping unit or the piezoelectric unit may be omitted under some conditions, since if the device is being operated under conditions which favor a constant rate of heat loss from the device, then the heating units, or the heating circuit,

or both, may be designed so that with continuous operation, just enough heat is supplied to hold the temperature of the damping unit or piezoelectric unit at the desired level.

Various applications of the basic features of the invention as disclosed above will be apparent to those skilled in the art, it being obvious that the benefits to be derived from the use of a heated piezoelectric unit either with or without heated damping means may be secured in whole or in part in numerous vibratory devices wherein the piezoelectric unit is operated either as a motor or as a generator. Furthermore, it will be apparent that while the principles underlying the invention are lapplicable particularly to Rochelle salt units, they are equally applicable to other materials of similar nature which exhibit one or more characteristics of the type disclosed herein. Reference is made especially to the rather extensive class of tartrates which are isomorphous with Rochelle salt. Having now disclosed our invention, what we claim is:

1. AA piezoelectric transducer including in comv bination a vibratory mechanical system coupled to an artificially heated piezoelectric unit containing piezoelectric material having substantially the properties of Rochelle salt, said unit having when in operation a temperature above the upper Curie point of said piezoelectric material and below the temperature at which said material becomes piezoelectrically inoperative.

2. A piezoelectric transducer as claimed in claim 1 which includes artificially heated damping means embodying material whose damping properties vary with temperature, said damping means having, when in operation, a temperature above the ambient room temperature.

3. A piezoelectric transducer as claimed in claim 1 wherein said piezoelectric unit contains Rochelle salt, and its temperature when in operation is between 30 C. and 45 C.

4. A piezoelectric transducer as claimed in claim 1 wherein said piezoelectric unit contains Rochelle salt, and its temperature when in operation is around 30 C.

5. A piezoelectric transducer including in combination, a vibratory mechanical system, an artificially heated piezoelectric unit containing Rochelle salt, and artificially heated mechanical damping means containing damping material whose damping properties vary with temperature,l

6. The method of improving the operating characteristics of a piezoelectric transducer which includes as elements thereof in combination a vibratory mechanical member coupled to a piezoelectric unit which contains piezoelectric material having substantially the properties of Rochelle salt, said method comprising the step of operating said unit at a temperature above the upper Curie point of said piezoelectric material and below the temperature at which said piezoelectric material becomes piezoelectrically inoperative.

7. The method as claimed in claim 6, wherein said piezoelectric material is Rochelle salt, and said temperature is between about 30 C. and about 45 C.]

8. In a piezoelectric transducer, the combination of a vibratory mechanical system, a piezoelectric unit coupled to said system. said piezoelectric unit containing piezoelectric material having substantially the characteristics of Rochelle salt, electrical heating means adapted to supply heat to said unit, and temperatureresponsive means adapted to control said heating means to maintain the temperature of said unit above the upper Curie point of said piezoelectric material. 1,

9. In combination, a piezoelectric unit comprising piezoelectric material having substantially the characteristics of Rochelle salt, a member coupled to said piezoelectric unit and adapt-- ed to be vibrated thereby, frequency-selective damping means coupled to said member and adapted to dissipate vexcess energy therefrom at selected frequencies of vibration, heating means adapted to supplyvheat to said piezoelectric unit and damping means, and temperature-responsive means adapted to control said heating means to maintain the temperature of said unitl above the upper Curie point of said piezoelectric material, and to maintain the temperature of said damping means at least above the ambient room ytemperature.

10. In a piezoelectric transducer, the combination of a vibratory mechanical system, a piezoelectric unit coupled to said system, said piezoelectric unit containing piezoelectric material having substantially the characteristics of Rochelle salt, a pair of electric heating units disposed in heat exchange relationship adjacent opposite faces of the piezoelectric unit, and thermostatic means disposed in the vicinity of one of said heating units and adapted to control said heating means to maintain the temperature of' said piezoelectric unit above the upper Curie point of said piezoelectric material.

11. In combination, a piezoelectric unit comprising piezoelectric material having substantially the-properties of Rochelle salt, a vibratory mechanical member coupled to said piezoelectric unit to produce an electro-mechanical system adapted to interconvert electrical and mechanical vibratory energy, and means for heating said piezoelectric unit to above the upper Curie point of its said piezoelectric material and below the temperature at which the material loses its piezoelectric properties.

12. In combination, a piezoelectric unit comprising piezoelectric material Ahaving substantially the characteristics of Rochelle salt, a mechanical vibratory member coupled to said piezoelectric unit to produce an electro-mechanical system adapted to interconvert vibratory electrical and mechanical energy, damping means associated with said system and' adapted to absorb energy therefrom, and heating means adapted to heat said vpiezoelectric unit and damping means to above the upper Curie point 'of said piezoelectric material.

13. The combination as claimed in claim. l2 including temperature-responsive means for controlling said heating means to maintain'the temperaturel of said piezoelectric unit above the ambient temperature and above the upper Curie point, but not inexcess of the temperature at which the piezoelectric material loses its piezoelectric properties, and to maintain the temperature of said damping means above the ambient temperature.

14. Piezoelectric apparatus comprising a piezoelectric unit having greater length and width than thickness and operatively supported in housing means therefor, a pair of electrical heating units supported by said housing'means and disposed on opposite sides of saidfunit adjacent perature of said damping means above the ambient room temperature and to maintain the temperature of the piezoelectric unit above the upper Curie point of its piezoelectric material but below the temperature at which said material becomes piezoelectrically inactive.

15. In combination, a piezoelectrically-actuated vibratory system comprising an artificially heated piezoelectric unit containing piezoelectric material having substantially the characteristics of Rochelle salt, said unit being at a temperature above the upper Curie point when in operation, a signal source energizing said piezoelectric unit and incapable of overloading said unit at the operating temperature, the impedance of said source being sufilciently high to prevent overloading of lsaid unit at temperatures below said operating temperature.`

16. In a piezoelectric device, the combination of: a piezoelectric unit comprising piezoelectric material having substantially the properties of Rochelle salt; a vibratory mechanical member coupled to said piezoelectric unit; and means for heating said piezoelectric unit to above the upper Curie point of its said piezoelectric material and below the temperature at which the material loses its piezoelectric properties:

CHARLES K. GRAVLEY. JOSEPH J. NEFF. 

